1. Field of the Invention
The present invention relates to a power storage device and a method for manufacturing an electrode.
Note that a power storage device in this specification refers to every element or device which has a function of storing electric power.
2. Description of the Related Art
In recent years, a variety of power storage devices such as lithium secondary batteries, lithium-ion capacitors, and air cells have been developed. In particular, a lithium secondary battery in which charge and discharge are performed by transfer of lithium ions between a positive electrode and a negative electrode has been attracting attention as a secondary battery with high output and high energy density.
An electrode used for a power storage device is manufactured by forming an active material layer over one surface of a current collector. The active material layer is formed using an active material such as carbon or silicon, which can store and release ions serving as carriers. For example, when an active material layer is formed using silicon or phosphorus-doped silicon, the theoretical capacity is higher than that in the case where the active material layer is formed using carbon, which is advantageous in increasing the capacity of a power storage device (see Patent Document 1).
However, it is known that the volume of silicon serving as an active material is expanded when lithium ions are inserted and contracted when lithium ions are extracted. Therefore, a problem arises in that an active material layer is powdered and detached from a current collector along with charge and discharge, for example. As a result, the current collecting property in an electrode is decreased and the charge-discharge cycle characteristics are degraded. As a countermeasure against this, there is a method in which a surface of an active material layer is coated with carbon, copper, nickel, or the like to suppress powdering and separation of silicon. However, such coating may inhibit reaction between lithium and silicon and may reduce the charge-discharge capacity.